8.9 Stability and coagulation of colloids

1) Stability of colloids Colloids, a dispersion system with high specific area and thus high interfacial specific energy, is thermodynamically unstable. Collision between colloidal particles frequently occur and aggregation is always a possibility.

Stabilizing factors: Dynamic stabilization: Brownian motion and diffusion prevents colloidal particles from sedimentation. Electric stabilization: All the colloid particles in a particular system have the same charge tends to repulse each other and keep the colloid in suspension. The interaction between particle and solvent also helps to prevent aggregation of colloidal particles on collision.

2) DLVO theory Around 1942, Deijaguin-Landao-Verwey-Overbeek 1) The inter-particle attraction: long-range dispersion forces Hamaker constant 2) The inter-particle repulsion: Debye-Hückel constant

The inter-particle potential x E E x Potential curves for colloidal systems with different . Interparticle potential curve

 r  /V Hamaker constant 0.03 0.00 As the concentration of electrolyte increases,  of colloid decreases. When  = 0.03 V, colloid begins to settle. When  = 0, precipitation rate attains maximum.

3) Precipitation of colloids by electrolytes A lyophobic colloid is very sensitive to the addition of a small amount of an electrolyte. The electrolyte causes a compression of the diffusion layer of the double layer and decreases electrokinetic potential, which allows two particles to make a closer approach to each other. Precipitating value and precipitating efficiency / power Precipitating value: The lowest concentration of electrolyte (usually in mmol dm-3) at which precipitation of colloid can be easily observed.

Precipitating value of different electrolytes towards the same colloids As2S3 () Al(OH)3(+) LiCl 58 NaCl 43.5 51 KCl 46 49.5 KNO3 60 CaCl2 0.65 K2SO4 0.30 MgCl2 0.72 K2Cr2O7 0.63 MgSO4 0.81 (KOOC)2 0.69 AlCl3 0.093 K3[Fe(CN)6] 0.08 Al(NO3)3 0.095

the higher the valence, the lower the precipitating value. The ion which is effective in causing precipitation of a sol is the one whose charge is of opposite sign to that of the colloidal particles, i.e., counterions Hardy and Schulze made systematic investigation on the precipitation of sols by adding electrolytes and summarized rules latterly named as Hardy-Schulze rules. (1) Valence: the higher the valence, the lower the precipitating value. Hardy-Schulze rules is only valid without specific adsorption. The precipitating efficiency of morphia (I) chloride is larger than Mg (II) and Ca (II) DLVO suggests

Nonregular aggregation (2) radius Hofmeister / lyotropic series H+ > Cs+ > Rb+ > NH4+ > K+ > Na+ > Li+ F- > Cl- > Br- > NO3- > I- (3) co-ions When counterion is the same, the higher the valence of the co-ions, the higher the precipitating value. Precipitating values for As2S3 colloids Electrolytes KNO3 ½ K2SO4 Precipitating values 50 65.5 Nonregular aggregation Fe(OH)3 sol + HCl

aggregation of sols by electrolytes 1) Preparation of soy-bean curd. 2) Detoxification of heavy metal ions 3) Formation of delta and alluvion When the river water containing colloidal clay flows into the sea, the brine induces coagulation. This is a major cause of silting in estuaries.

4) mutual precipitation of colloids Generally, mixing of colloids with the same charge does not lead to precipitation, while mixing of colloids with different charge will result in mutual precipitation. Fe(OH)3(3.04 g dm-3) 9 8 7 5 3 2 1 0.2 As2S3(2.07 g dm-3) 9.8 charge + - phenomenon --- Turbid  Purification of water using alum

Coagulation Coagulation is the process of adding chemicals to water to make dissolved and suspended particles bind together (coagulate) and form larger particles (flocculant) that settle out of the water. Aluminum sulphate, ferric sulphate, ferric chloride, and forms of aluminum or iron salts called polymers are all suitable coagulation chemicals approved for water treatment.

5) effect of macromolecules on colloids 1) Stabilization effect: When lyophilic sol, such as gelatin, albumin, agar, casein, gum arabic, glue, starch, etc. is added to a sol, the latter may be prevented from precipitation by electrolytes. Such macromolecules are named as stabilizing agent or stabilizers. The macromolecules adsorbed on the colloidal particle form a tough shell which helps to keep the particle apart.

Gold number: the number of milligrams of the protective colloid that just prevents the change of color when 1 cm3 of the standard salt solution (10 % NaCl) is added to 10 cm3 of the standard gold sol (0.006 %). (By Zsigmondy) Protective colloids Gold number / mg Gelatin (明胶) 0.005-0.01 Albumin (白蛋白) 0.1-0.2 Gum arabic 0.15-0.5 Dextrin (糊精) 6.0-20.0 Potato starch 25 Red number: Congo red

2) Sensitization effect: When small amount of some lyophilic sols was added into a lyophobic sol, the sol can be precipitated by less amount of an electrolyte. In other words, the addition of the lyophilic sols decrease the precipitating value of the sol. This phenomenon is named as sensitization. LaMer: Bridging effect The recoil of the macromolecules helps to draw several particles together.

Flocculation of colloids: flocculants Chemicals by adding which suspended solids in the wastewater are sedimented and clearized water is gotten. Kinds of flocculants: 1. inorganic: polyalminum chloride 2. polymers: polyacrylic acid, polyacrylamide derivatives 3. naturally occurring flocculants: chitosan, sodium arginate

Polymeric electrolyte – polyacrylamide (PAM) -[- CH2 – CH-]m- C=O NH2 M > 106 Differences in flocculation and aggregation flocculants are widely used in water treatment plant, pulp industries ,the food industries and so on.,

Coagulation treatment improved the water quality by 1. reducing the dissolved organic carbon (DOC) concentration by about 60%; 2. reducing color by over 80%; 3. reducing the dissolved phosphate concentrations by as much as 99%; 4. reducing nutrient availability; 5. reducing the alkalinity and pH of the water.

In general, the combined effect of these changes can: 1. reduce algae growth and limit algae blooms, therefore limiting the risks from toxic blue-green algae; 2. make the water easier to treat for other purposes such as domestic use; 3. make the water safer for chlorination due to decreased levels of DOC and therefore, decreased disinfection by-products (trihalomethanes).